As printing technology advances, manufacturers of many different types of products are faced with the increasingly rigorous demands of their customers for high quality printed images on those products. Such products include, for example, printed paper, printed polymeric sheets, coatings or films, printed metallic items, and the like.
For example, there is a keen demand for paper that can be printed on to provide images of exceptionally high quality, particularly with respect to brightness, clarity, opacity, water-fastness, water resistance, bleed resistance and rub resistance. The customer further demands that paper be amenable to use with a variety of printing techniques, including not only conventional printing techniques, but also "impact-free" printing techniques such as inkjet printing (particularly colored inkjet printing), laser printing, photocopying, and the like.
In response, paper manufacturers have attempted to meet their customers' demands for such high quality paper through a process termed "sizing." "Sizing," which encompasses both "internal sizing" and "external sizing," affects the manner in which colorants and particularly ink interact with the fibers of the paper. "Internal sizing" involves introduction of sizing compositions within the entire fibrous mass at the pulp stage of paper manufacturing (i.e., to the wet pulp, or more specifically, to the prepared papermaking furnish) before the stock is formed into a sheet, resulting in the distribution of the sizing composition within the entire fibrous mass that is subsequently used to produce the flat fibrous paper sheet. "External sizing" (also referred to as surface application, pasting, saturating or coating) involves application of a sizing composition to at least one surface of a fibrous paper sheet, so that the composition is present on or in at least one of the two faces of the fibrous sheet. Various materials have been used as sizing agents, such as conventional and modified starches, polyvinyl alcohol, cellulosic derivatives, gelatin, rosin, proteins such as casein, natural gums and synthetic polymers. Although these materials are effective to various degrees under certain conditions, use of each is associated with certain limitations. For example, it is often necessary to use large amounts of these conventional sizing agents in order to provide paper having the desired properties. However, the opacity and brightness of the paper substrate decrease in direct proportion to the amount of sizing agent applied to the paper. Moreover, as the amount of sizing agent and/or the cost of the sizing agent increases, the cost of producing the paper increases, making high quality papers prohibitively expensive. Certain sizing agents impart relatively poor bleed resistance and water resistance of imprinted inks, and thus must be used with insolubilizing agents to ensure production of a printed paper having satisfactory water resistance.
Use of conventional sizing agents also results in a decrease in the porosity of the final paper substrate; thus, while the sized paper substrate may have the desired brightness and opacity, it may not provide for a printed image having a suitable optical density or color intensity. In addition, as the porosity of the paper increases, the paper becomes less amenable to various handling processes during manufacturing. For example, envelope manufacturers demand that the paper available to them have a relatively low porosity. If the porosity of the paper is too high, the paper is too stiff for handling by automated industrial devices for folding and sorting (e.g., devices of the "suction extractor" type) during envelope production. In contrast to lower porosity papers, high porosity papers also require slower machine speeds, and further require refining and draining operations that have relatively high energy costs.
Coatings have additionally been used to enhance the quality of printed images on paper, albeit with limited success. Regarding other types of substrates, a variety of coating methods and compositions have also been suggested. However, as with paper, there is not at this time any satisfactory method for improving the quality of images printed on polymeric films, metallic sheets, or the like using water-based inks.
The present invention is directed to the aforementioned need in the art, and provides an efficient, versatile and cost-effective means for treating substrates that can then be printed on to yield high quality, water-resistant printed images. The compositions and methods of the invention are amenable for use with a wide variety of substrate types, and are compatible with conventional manufacturing and post-manufacturing handling processes.